Branched Poly(Aryl Piperidinium) Membranes for Anion‐Exchange Membrane Fuel Cells

Abstract Anion‐exchange membrane fuel cells (AEMFCs) are a promising, next‐generation fuel cell technology. AEMFCs require highly conductive and robust anion‐exchange membranes (AEMs), which are challenging to develop due to the tradeoff between conductivity and water uptake. Here we report a method to prepare high‐molecular‐weight branched poly(aryl piperidinium) AEMs. We show that branching reduces water uptake, leading to improved dimensional stability. The optimized membrane, b‐PTP‐2.5, exhibits simultaneously high OH− conductivity (>145 mS cm−1 at 80 °C), high mechanical strength and dimensional stability, good processability, and excellent alkaline stability (>1500 h) in 1 M KOH at 80 °C. AEMFCs based on b‐PTP‐2.5 reached peak power densities of 2.3 W cm−2 in H2−O2 and 1.3 W cm−2 in H2‐air at 80 °C. The AEMFCs can run stably under a constant current of 0.2 A cm−2 over 500 h, during which the b‐PTP‐2.5 membrane remains stable.


Synthesis of model polymer 1: hyperbranched poly(triphenylbenzene piperidine)
In a flask, TPB (4.696 g, 1 equiv.) and N-methyl-4-piperidone (1.734 g, 1 equiv.) were added into DCM (17.45 mL). The solution was stirred with overhead agitator under 0 o C for 30 min. Then TFA (1.22 mL, 1.1 equiv.) and TFSA (13.61 mL, 10 equiv.) were added dropwise to the solution. During reaction, the color of the solution changed from light yellow to brick red. The resulted viscous solution was poured into excess amount of methanol. The white product was filtered and washed with 1M K2CO3 at 50 o C overnight, followed by washing three time with water and dried in oven at 80 o C under vacuum for 24 h. The obtained Model Polymer 1 is soluble in DMF, NMP, THF and CHCl3.

Synthesis of model polymer 2: branched poly(benzene piperidine)
In a flask, benzene (1.109 g, 0.95 equiv.), TPB (0.229 g, 0.05 equiv.) and 1-methyl-4-piperidone (1.734 g, 1.025 equiv.) were added into DCM (17.45 mL). The solution was stirred with overhead agitator under 0 o C for 30 min. Then TFA (1.22 mL, 1.1 equiv.) and TFSA (13.61 mL, 10.25 equiv.) were added dropwise to the solution. The resulted viscous solution was poured into excess amount of methanol. The white product was filtered and washed with 1M K2CO3 at 50 o C overnight, followed by washing three time with water and dried in oven at 80 o C under vacuum for 24 h. The obtained model polymer 2 is soluble in NMP, THF and CHCl3.

Synthesis of model polymer 3: branched poly(biphenyl piperidine)
In a flask, BP (2.190 g, 0.95 equiv.), TPB (0.229 g, 0.05 equiv.) and N-methyl-4-piperidone (1.734 g, 1.025 equiv.) were added into DCM (17.45 mL). The solution was stirred with overhead agitator under 0 o C for 30 min. Then TFA (1.22 mL, 1.1 equiv.) and TFSA (13.61 mL, 10.05 equiv.) were added dropwise to the solution. During reaction, the color of the solution changed from light yellow to brick red. The resulted viscous solution was poured into excess amount of methanol. The white product was filtered and washed with 1M K2CO3 at 50 o C overnight, followed by washing three time with water and dried in oven at 80 o C under vacuum for 24 h.

Synthesis of branched poly(terphenyl piperidinium) (b-PTP-x)
In a flask, b-PTPA-x (1 g) was suspended in DMSO (30 mL).and the solution was stirred at room temperature for 30 min. Subsequently, K2CO3 (0.39 g) and iodomethane (1 mL) were added and the reaction was kept stirred at room temperature for 24 h in the dark. Ethyl acetate was added to the resulting viscous solution. The light-yellow precipitate was filtered, washed three times with water, and dried in oven at 80 o C under vacuum for 24 h.

Membrane preparation
b-PTP-x (1 g) was dissolved in 30 mL DMSO and the polymer solution was filtered through a 0.45 µm polytetrafluoroethylene (PTFE) filter and casted onto a clean glass plate. Subsequently, the solution was evaporated at 80 o C for 12 h, 120 o C for 12 h, and dried at 120 o C under vacuum for 24 h to completely remove the residual solvent. The membrane in Iform was peeled off from glass plate. Membrane in Brform was obtained by ion exchanging in 1 M KBr solution at 80 o C for 12 h, followed by washing with deionized water for 3 times to remove residual salts. Membrane in OHform was obtained by ion exchanging in 1 M KOH solution at 80 o C for 12 h, followed by washing with degassed deionized water for 3 times under N2 atmosphere. Membrane in OHform was stored under N2 to avoid contamination by CO2 and the formation of carbonate.

Intrinsic viscosity
The intrinsic viscosity ([η]) was determined by using an Ubbelohde viscometer. b-PTP-x was dissolved in DMSO into four different concentrations. The efflux time of each concentration was recorded four times at 22 o C. The reduced viscosity (ηred) and inherent viscosity (ηinh) were calculated by: where tb (s) is the efflux time of DMSO, ts (s) is the efflux time of the polymer solution at concentration c (g dL -1 ). The intrinsic viscosity ([η]) was obtained by extrapolating ηred and ηinh to c=0 and the average value was adopted.

H nuclear magnetic resonance spectroscopy
The 1 H spectra of protonated b-PTPA-x and model polymers, as well as the b-PTP-x, were measured by a Bruker Avance 400 spectrometer using DMSO-d6 with 5% vol% TFA. By adding TFA, the peak of water was shifted so that the peaks of piperidine/piperidinium ring could be observed.

Scanning electron microscopy
Surface morphology of dry b-PTP-x membranes in Iform was observed with a Zeiss Merlin microscope at 1 kV and equipped with an Inlens secondary electron detector. Membrane samples were coated with gold before analysis.

Thermal and mechanical properties
The thermal properties of b-PTPA-1 powder, protonated b-PTPA-1 powder, b-PTP-1 (Iform) powder, as well as b-PTP-x membranes (Iform) were measured by a Perkin Elmer TGA 4000 under N2 atmosphere. Samples were hold at 120 o C for 10 min to remove adsorbed water, and then heated from 50 o C to 800 o C with a heating rate of 10 o C min -1 .
The mechanical properties of b-PTP-x membranes were measured by Universal Testing Machine (UTM; AGS-500NJ, Shimadzu, Tokyo, Japan) at room temperature. Membrane samples were cut into a dumbbell-like shape with effective area of 2 mm × 10 mm and stretched at 1 mm min -1 .

Dynamic mechanical analysis
The storage modulus and tan δ of PTP and b-PTP-2.5 membranes in Iform were measured by a dynamic thermomechanical analysis (DMA, Q800. TA instrument, DE, USA) system. Membrane samples were cut into 9 mm × 20 mm rectangle shape and then measured with a preload force of 0.01 N and a force track of 125% under N2 atmosphere. The sample was ramped at 4 o C min -1 until 450 o C. The peak of tan δ stands for the glass transition temperature (Tg) of membrane sample.

IEC measurement
The ion exchange capacity (IEC) of b-PTP-x membranes was measured by Mohr titration. Weights (Wdry) of membrane samples in Brform were recorded after drying in an oven under vacuum at 80 o C overnight. Then, the samples were ion exchanged with 0.1 M NaNO3 at 50 o C for 12 h and repeated three times. The solutions were combined and titrated by 0.01 M AgNO3 using K2CrO4 as indicator. The IEC (mmol g -1 ) of b-PTP-x membranes in Brand OHform can be calculated by: where VAgNO3 is the volume (mL) of consumed AgNO3 solution.

Water uptake and swelling ratio
Dry weight (Wdry) and length (Ldry) of b-PTP-x membranes samples were recorded after drying in an oven at 80 o C under vacuum. Then the samples were immersed in degassed deionized water for 12 h at different temperatures under N2. The wet weight (Wwet) and length (Lwet) of the samples were recorded after wiping excessive water from the surface. The water uptake (WU) and swelling ratio (SR) can be calculated by: The hydration number (λ) is the number of adsorbed water molecules per piperidinium group. It can be calculated by: where M(H2O) is the relative molecular weight of water (18 g mol -1 ).

Conductivity measurement
The in-plane ion conductivity (mS cm -1 ) of b-PTP-x membranes was measured by an Autolab PGSTAT302N equipped with a Scribner 740 MTS. Membrane samples in 10 mm × 30 mm were assembled into a four-electrode cell and tested using the alternative current (AC) impedance in the frequency range of 1 Hz to 0.1 MHz. The assembly and measurement were operated under N2 to avoid CO2 contamination. The measurements were carried out at different temperature at 100% RH. The ion conductivity σ (mS cm -1 ) of membrane samples can be calculated by: σ = (8) where L (cm) is the distance between working and counter electrode, A (cm 2 ) is the cross-sectional area calculated by the thickness (cm) and width (cm) of the membrane samples, R (kΩ) is the Ohm impedance obtained from electrochemical impedance spectra.

Gas permeability
The H2 permeabilities of PTP, b-PTP-2.5, commercial FAA-3-50 and Nafion 212 membranes were measured by a gas permeability testing system equipped with gas chromatography (GC, 490 Micro GC, Agilent Technologies, USA) and two mass flowmeters (MFC, M3030V, Line Tech, Korea). [1,2] The measurements were conducted under different RHs with 2.2 bar unilateral backpressure at 80 o C. Gas permeability P (Barrer, 1 Barrer = 10 -10 cm 3 (STP) cm cm -2 s -1 cmHg -1 ) can be calculated by: where V (cm 3 ), Mgas (g mol -1 ), d (um) and Pfeed (760 mmHg) are the volume of measurement cell, molecular weight of permeating gas, membrane thickness and the pressure of feed gas, and R (L mmHg K -1 mol -1 ), T (K), A (cm 2 ) and ρ (g cm -3 ) are the gas constant, testing temperature, effective area of samples and the density of permeating gas, respectively. denotes the change rate of permeated gas pressure over time.

Alkaline stability
The alkaline stability of b-PTP-2.5 membrane was measured by immersing into 1 M and 3 M KOH at 80 o C over 1500 h. The KOH solution was refreshed weekly. Before testing the conductivity, the sample was washed three times with degassed deionized water under N2. To measure 1 H NMR spectra, surface morphology and mechanical property, the sample was ion exchanged with 1M KI at 80 o C for 12 h, followed by washing three times with deionized water and drying.

Fuel cell performance
The single fuel cell performance was measured by a fuel cell station (CNL, Seoul, Republic of Korea). The anode slurry was prepared by adding 12.5 mg Pt-Ru/C (40 wt% Pt, 20 wt% Ru, Hispec, Alfa Aesar, USA) and 5 mg poly(fluorenyl-co-biphenyl piperidinium) (PFBP, 5 wt% in DMSO) ionomers to isopropanol/water (1.2 mL/0.12 mL) solution. The composition of ionomer: carbon: Pt-Ru is 1: 75: 1.5. The cathode slurry was prepared by adding 12.5 mg Pt/C (40 wt% Pt, Hispec, Alfa Aesar, USA) and 3.75 mg PFBP ionomer to isopropanol/water (1 mL/0.1 mL) solution. The composition of ionomer: carbon: Pt is 1: 2: 1.33. The slurries were sonicated for 45 min at 0 o C before spraying. Then, the anode and cathode slurries were sprayed onto both sides of b-PTP-2.5 membranes (Iform) along with metal catalyst loadings of 0.6 and 0.4 mg cm -2 , respectively. The prepared catalyst-coated membrane (CCM) was immersed in 1 M NaOH at room temperature for 12 h, followed by washing three times in deionized water. After that, the CCM was assembled with gas diffusion layers (GDLs), PTFE gaskets and graphite bipolar plates into single cell at a torque of 60 in-lb. The effective area of the cell is 5 cm 2 .
The assembled cell was connected with gas lines and heating system. After the temperature of humidifier and gas lines reached the set values, cell temperature was increased to 70 o C. The cell was activated at a constant voltage of 0.5 V at flowrate of H2 and O2 at 1000 mL min -1 until no fluctuation of current density. After that, the temperature of cell was increased to 80 o C and the polarization curve was recorded.
The in-situ durability of fuel cell was measured under a constant current density of 0.2 A cm -2 at 60 o C with 400/400 mL min -1 H2/O2 flowrate with Anode (A)/Cathode (C) dew points of 53 o C and 60 o C. The cell was refreshed two times during durability test. After durability test, the membrane was disassembled from the MEA and used for 1 H NMR, surface morphology and mechanical property measurements.

Results and Discussion
" Figure S1. DLS intensity-based size distribution for PTP and b-PTP-x.
The linear PTP reference exhibited one DLS peak at around 14 nm. This peak was shifted to higher size and its intensity was decreased with increasing branching degree. Meanwhile, another peak at around 100 nm appeared for the branched polymers. The size and intensity of the latter peak increased with increasing branching degree. Although the interpretation of the DLS spectra of b-PTP-x is difficult, the data are consistent with a higher molecular weight upon branching.